Looks like keeping them completely separate is the safe and sure bet. 

So far I've devoured books, blogposts and youtube videos on different topics, but one that I'm not yet 100% comfortable is the methodology on finding a market to invest on.I live in CA and I think the prices are crazy high right now, so I'm evaluating other area, but need a good framework to select it based on my investing goals.What would you say are good techniques to pick a market to invest on?

Having an LLC helps to keep your personal property (including your W2 pay) safe from a lawsuit. 

On the 4plex I would recommend having it inspected to see if the systems are in good shape, and getting some contractor quotes on what repairs are necessary to make it safe and functional.

Stay safe  

Nowhere is safe from Mother Nature, every place has some exposure to risk from natural disasters (Midwest has tornados, the west has wildfires, volcanos, earthquakes, east coast has hurricanes, etc.).

We got a step by step here on all the things wrong with the techniques that have earned money time out of mind, and we got a list of benefits for your approach.

@Xiao XiongHi Xiao, as a real estate agent, it would be illegal & unethical to tell you what I think is a safe area or not.

We rented previously and purchased in Connecticut, smaller city - and safe. 

More specifically I want to gather information on a unique real estate investing technique called military house hacking, or using a VA loan to invest in real estate.